The operating temperature within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature capabilities have been achieved through the development of iron, nickel and cobalt-based superalloys and the use of oxidation-resistant environmental coatings capable of protecting superalloys from oxidation, hot corrosion, etc.
In the compressor portion of an aircraft gas turbine engine, atmospheric air is compressed to 10–25 times atmospheric pressure, and adiabatically heated to 800°–1250° F. in the process. This heated and compressed air is directed into a combustor, where it is mixed with fuel. The fuel is ignited, and the combustion process heats the gases to very high temperatures, in excess of 3000° F. These hot gases pass through the turbine, where rotating turbine wheels extract energy to drive the fan and compressor of the engine, and the exhaust system, where the gases supply thrust to propel the aircraft. To improve the efficiency of operation of the aircraft engine, combustion temperatures have been raised. Of course, as the combustion temperature is raised, steps must be taken to prevent thermal degradation of the materials forming the flow path for these hot combustion gases.
Every aircraft gas turbine engine has a so-called High Pressure Turbine (HPT) to drive its compressor. The HPT sits just behind the combustor in the engine layout and experiences the highest temperature and pressure levels (nominally 2400° F. and 300 psia respectively) developed in the engine. The HPT also operates at very  high speeds (10,000 RPM for large turbofans, 50,000 for small helicopter engines). In order to meet life requirements at these levels of temperature and pressure, HPT's today are always air-cooled and constructed from advanced alloys.
While a straight turbojet engine will usually have only one turbine (an HPT), most engines today are of the turbofan or turboprop type and require one or two additional turbine(s) to drive a fan or a gearbox. This is called the Low Pressure Turbines (LPT) and immediately follows the HPT in the engine layout. Since substantial pressure drop occurs across the HPT, the LPT operates with a much less energetic fluid and will usually require several stages (usually up to six) to extract power.
Components formed from iron, nickel and cobalt-based superalloys cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the LPT and HPT sections. A common solution is to provide such components with an environmental coating that inhibits oxidation and hot corrosion and a thermal barrier coating to improve maximum operating temperature. Coating materials that have found wide use for this superalloy generally include diffusion aluminide coatings and thermal barrier coatings. The ceramic thermal barrier coatings (TBCs) are generally formed by such methods as physical vapor deposition (PVD). The diffusion aluminide coatings are generally formed by such methods as diffusing aluminum deposited by chemical vapor deposition (CVD) or slurry coating, or by a diffusion process such as pack cementation, above-pack, or vapor (gas) phase aluminide (VPA) deposition into a superalloy substrate. The diffusion aluminide coatings can serve as bond coats to promote adhesion of the TBCs and, with an alumina scale layer on the surface of the diffusion aluminide coating, can serve as diffusion barriers for the metals in the TBCs.
A diffusion aluminide coating generally has two distinct zones, the outermost of which is an additive layer containing an environmentally resistant intermetallic generally represented by MAI, where M is iron, nickel, or cobalt, depending on the substrate material. Beneath the additive layer is a diffusion zone comprising various intermetallic and metastable phases that form during the coating  reaction as a result of diffusional gradient and changes in elemental solubility in the local regions of the substrate. During high temperature exposure in air, the outer surface of the additive layer forms a protective aluminum oxide (alumina) scale or layer that inhibits oxidation of the diffusion coating and the underlying substrate. Currently, many aircraft engine components, including HPT and LPT components are protected from the environment with TBCs. Greater performance is being sought for these systems to increase turbine temperature capability and improve reparability of components. However, TBC performance is limited by the thermal capability of diffusion aluminide coatings acting as bond coats. This limitation highlights the need for cost-effective processes to apply coatings with better thermal capabilities.
In addition to the limitations of TBC performance, turbine engine components with airfoils generally use film cooling in which relatively cooler air is forced though cooling passages in the components. These internal cooling passages do not require coatings in addition to the aluminide as the surfaces of such tubes are generally not exposed to the same high temperatures as the outer surfaces of the components. In addition, the internal surfaces are not accessible to many types of coating techniques, such as those employing line-of-sight deposition processes. The protective layer on the internal surfaces cannot be readily repaired, and therefore must last longer than the protective layer on the external surfaces, which can be refurbished. Additionally, the internal surfaces are subjected to a significantly different service environment than the external surfaces. The external surfaces experience hot corrosion, hot oxidation, and erosion in the combustion gas. On the other hand, a flow of bleed air from the engine compressor, not combustion gas, is passed through the internal passages, and the internal surfaces are at a lower temperature than the external surfaces. The bleed air typically may contain, salt, sulfur, and other corrodants drawn into the compressor of the engine. The presence of the combination of salt and sulfur at a temperature in the range of about 1300° F. (X° C.), a typical temperature for the internal surfaces, may lead to severe Type II corrosion on the internal surfaces. The internal surfaces are additionally subjected to low-to-medium temperature oxidation. The internal surfaces of the gas turbine  components are thus subjected to environmental damage of a type substantially different from that experienced on the external surfaces.
What is needed is a modified bond coat with higher thermal capabilities that can be applied to an article with a low cost process, while simultaneously applying an aluminide coating to the internal surface of the article.